1. Field of the Invention
The present invention relates to a spectroscopic ellipsometer for measuring the thickness of a thin film on surfaces of a target sample, such as a semiconductor wafer, a reticle/mask, a glass substrate of a liquid crystal display (LED) and the like, and more particularly for permitting a precise measurement of a selected area on a target surface.
2. Description of Related Art
A spectroscopic ellipsometer observes a change in the polarization state of light which reflects from the surface of a substance in order to measure optical constants (refractive index, extinction coefficient) of the surface, and if a thin film layer also exists on the surface of the substance, it can measure the film thickness and the optical constants of the thin film layer.
In a conventional spectroscopic ellipsometer, a F-number in an irradiating optical system has been equalized with a F-number in a detecting optical system, and the distribution range of the incidence angle of an irradiating light contacting the surface of the sample has been relatively wide. Since the spectroscopic ellipsometer determines objective values through an arithmetic processing based on the angle of incidence of the irradiating light, and a polarization characteristic of the irradiating light and a reflected light and the like, measurement to a high degree of precision becomes difficult if the distribution range of the incidence angle of the irradiating light is wide.
If, for this situation, the incidence angle of the irradiating light is intended to be kept constant, this can lead to the irradiation of a wide area of the surface of the sample, and therefore it is not possible to respond to a sample requirement of measuring only a relatively infinitesimal area.
In a spectroscopic ellipsometer, the angle of incidence of the irradiating light to the surface of the sample, a wavelength of the light and a polarization state are controlled, and the thickness, the refractive index (dielectric constant) and the like of the sample are estimated by arithmetic processing from the polarization characteristics of the reflected light and a reflection coefficient for each polarization component at the surface of the sample in each condition. Therefore, the angle of incidence of the irradiating light is an important controlling element from a measurement viewpoint and it is preferred that the irradiating light is a collimated light beam.
However, in actual fact, there are many situations that require measuring only a small area of the surface of the sample, and it is a general procedure to allow the irradiating light to have a certain magnitude of a solid angle to control a beam spot diameter of the irradiating light on the surface of the sample in such a case.
When light is condensed to a point using an optical system, a beam spot diameter d obtained is generally expressed by the following equation (1)d=A·λ·Fno.  (1)Wherein A is a constant, λ is a wavelength, Fno. is a F-number and the F-number is a magnitude which is given by f/D where the diameter of an entrance pupil of the optical system is expressed by D and a focal length of the optical system is expressed by f.
As understood from equation (1) described above, it is necessary to reduce the F-number in order to reduce a beam spot diameter d. In this case, a gradient of the beam will come to have a gradient corresponding to the magnitude of the F-number. When the beam spot diameter d is reduced in this manner, the distribution of the gradient of the irradiating light becomes wide, but the case where the distribution of the gradient of the irradiating light is smaller will provide a higher measurement precision when the reflected light is introduced to a detector to detect signals.
The present invention has been made in consideration of the matters described above, and an object of the present invention is to provide a spectroscopic ellipsometer capable of measuring an extremely small area with a high degree of precision.